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Nanostructured TiO2 architectures
for environmental and energy applications
Meidan Ye
a,b, Danny Vennerberg
a, Changjian Lin
b, Zhiqun Lin
a,c,*
a Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011, USA
b Department of Chemistry, College of Chemistry and Chemical Engineering,
Xiamen University, Xiamen 361005, China c School of Materials Science and Engineering, Georgia Institute of Technology,
Atlanta, GA 30332, USA
*Author for correspondence: Zhiqun Lin, email: [email protected]
Received 19 Mar 2011; Accepted 10 May 2011; Available Online 1 Jun 2011
Abstract
Semiconductor photocatalysts have important applications in the fields of renewable energy and
environmental remediation. Of these, TiO2 stands out as one of the most investigated compounds in contemporary
material science. Because of its unique physical and chemical properties, TiO2 is extensively used in
photoelectrochemical applications including photocatalysis and photovoltaics. In this Review, we focus especially
on recent progress in the fabrication of TiO2-based nanostructures. An overview of some important properties and
applications of TiO2 is given to provide context for the challenges that present research efforts aim to address. This
discussion is followed by a summary of synthesis routes currently available for the preparation of nanoscale TiO2.
Finally, the various nanoarchitectures reported to date are outlined with a particular focus on their utility in
photoelectrochemical applications.
Keywords: Nanostructured TiO2; Solar cells; Photocatalysis; Hydrogen generation; Sol-gel; Hydrothermal
1. Introduction
Titanium is the ninth most abundant
element in the Earth’s crust. Titanium dioxide
(TiO2), which is the most common compound of
titanium, is a typical n-type semiconductor.
Since the inception of its commercial production
in the early 20th
century, TiO2 has been widely
used in pigments, paint, and toothpaste. In 1972,
Fujishima and Honda observed the phenomenon
of photocatalytic water splitting by a TiO2
electrode under ultraviolet (UV) light [1]. This
discovery sparked intense research of TiO2 for
applications in photovoltaics, photocatalysis,
electrochromics, and sensors due to its excellent
physical and chemical properties, including
chemical stability, photostability and appropriate
electronic band structure [2-4].
Over the past decades, an exponential
increase in research effort has been devoted to
nanoscience and nanotechnology. This trend is
due, in part, to the amazing physical and
chemical properties that emerge when a material
is shrunk to nanometer scale dimensions, which
creates potential barriers for electrons within the
material, confining them to very small regions of
space. For semiconductor nanomaterials, the
movement of electron-hole pairs is primarily
governed by the well-known quantum
confinement effect [5]. Moreover, the
confinement can be in one dimension (producing
quantum films or quantum wells), in two
dimensions (producing quantum wires, tubes or
rods), or in three dimensions (producing
quantum dots (QDs)). These three regimes are
commonly referred to as 2D, 1D, or 0D,
respectively, although the terms are not so
precise [6-8]. Importantly, the specific surface
area and surface-to-volume ratio of a material
increase dramatically with decreasing size. High
surface area is beneficial to many nanomaterial-
based devices because it facilitates reaction or
interaction between the nanomaterial and
surrounding media. Thus, the performance of
these devices is strongly influenced by the sizes
of the material building blocks, especially at the
nanometer scale [5-9].
Environmental pollution and the need
for renewable energy are arguably the biggest
challenges of the 21st century. With world
populations booming, global energy demand has
continued to rise dramatically, exhausting our
fossil fuel supply at an alarming rate. A harmful
and unavoidable consequence of increasing
energy production based on fossil fuels is its
apparent impact on the environment. For
example, global warming resulting from fossil
fuel greenhouse gases threatens to severely alter
the climate. Additionally, the huge population
and fast economic development of emerging
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countries have introduced issues of pollution and
safe waste disposal, therefore, considerable
attention has been paid to these serious
challenges [10,11]. In order to meet the
increasing energy demand in an environmentally
acceptable manner, new sustainable materials
systems are necessary. Nanomaterials are
emerging as promising components in alternative
energy production because they offer attractive
properties while requiring fewer resources to
manufacture [12,13]. The enormous interest in
nanoscience during the past decade has
contributed to remarkable progress in
environmental remediation and renewable
energy technologies such as photocatalytic
oxidation, solar cells, fuel cells, and biofuels
[12]. The design and development of new
materials and substances that are chemically
modified from the molecular and atomic levels to
sizes on the nanoscale has yielded structures with
significantly improved properties for
environmental and energy applications [14].
As the most promising photocatalyst,
TiO2 is expected to play a key role in helping
ease energy shortages through effective
utilization of solar energy in photovoltaic
devices as well as solve many serious
environmental pollution challenges via
photocatalysis of pollutants. For this reason,
nanostructured TiO2 is one of the most intensely
researched substances, as is evident from the
more than 60,000 papers published about it over
the past 50 years (with a still robustly increasing
publication rate over the past decade) [4,15]. To
enhance its photocatalytic ability, various
morphologies of nanostructured TiO2 including
nanoparticles, nanorods, nanowires, and
nanotubes have been prepared, and much
progress on the synthesis of nanostructured TiO2
with excellent catalytic properties has been
made. In this Review, we focus on recent
developments in the fabrication of TiO2
nanostructures and present simplified principles
of some important synthesis methods.
2. Properties
In nature, TiO2 exists mainly in four
crystal phases: anatase, rutile, brookite, and
titanium dioxide (B) or TiO2 (B). Each phase
displays different physical and chemical
properties supporting different functionalities.
The detailed physical parameters of these four
crystal phases are listed in Table 1 [16]. In
general, anatase type TiO2 has a crystalline
structure that corresponds to the tetragonal
system (with dipyramidal habit), and rutile type
TiO2 also has a tetragonal crystal structure (with
prismatic habit). Brookite type TiO2 has an
orthorhombic crystalline structure, while TiO2
(B) has a monoclinic structure. Since TiO2 (B) is
a relative newcomer to the TiO2 family [17], the
majority of relative investigations have primarily
focused on the comparison among anatase, rutile
and brookite. Rutile is the most
thermodynamically stable phase for bulk TiO2
under most conditions. However, TiO2 generally
favors the anatase structure when prepared
through solution phase methods, which can be
attributed to two main effects: surface energy
and solution chemistry. At very small particle
dimensions (i.e. the nanoscale), the
transformation sequence among different TiO2
phases is size and pH dependent because the
energies of the four phases are apparently so
close to one another that they can be reversed by
small differences in surface energy [18,19]. All
other parameters equal, anatase is most stable at
diameters less than 11~14 nm, brookite is most
stable for crystal sizes between 14 and 35 nm,
and rutile is most stable for sizes exceeding 35
nm [20,21]. However, rutile is more stable than
anatase in very acidic solutions, whereas in very
alkaline solutions anatase is stabilized relative to
rutile and brookite [19]. In addition to being the
stable phase for bulk TiO2, rutile is the most
stable structure for nanoscale TiO2 at high
temperatures, though anatase and brookite are
commonly observed in natural and synthetic
nanocrystals. Under heating concomitant with
Table 1. Common TiO2 crystal phases and their structural parameters (Reprinted with permission from
Reference [16], Chen, S. S. et al., Chin. J. Catal. 2010, 31, 605-614. Copyright © Elsevier).
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coarsening, the following transformations are all
favored: anatase to brookite to rutile, brookite to
anatase to rutile, anatase to rutile, brookite to
rutile, and TiO2 (B) to anatase [4,16]. These
transformation sequences imply very closely
balanced energies as a function of particle size
[22]. Even the shape transformation of the TiO2
nanocrystals was found to be controlled by the
surface chemistry, as confirmed by theoretical
and experimental investigations [15,18,20,23].
Of the four phases, rutile and anatase
are the most practically important crystal
structures, and Figure 1 shows their unit cell
structures. These two structures can be described
in terms of chains of TiO6 octahedra, where each
Ti4+
ion is surrounded by six O2-
ions. The two
crystal structures are different in the distortion of
each octahedron as well as the assembly pattern
of the octahedra chains. That is, within rutile, the
octahedron exhibits a slight orthorhombic
distortion, whereas in anatase, the octahedron is
significantly distorted so that its symmetry is
lower than that of orthorhombic. Moreover, the
Ti-Ti distances of anatase are larger and the Ti-O
distances are shorter than those in rutile. In
assembling the rutile structure, each octahedron
is in contact with ten neighboring octahedrons
(two sharing edge oxygen pairs and eight sharing
corner oxygen atoms). In the anatase structure,
each octahedron couples to eight neighbors (four
sharing an edge and four sharing a corner)
[4,15,24,25]. Accordingly, these distinctions in
lattice structures produce differences in
electronic properties between the two phases of
TiO2. Generally, anatase appears to be the more
photoactive than rutile. This is probably because
of the differences in the extent and nature of the
surface hydroxyl groups present in the low
Figure 1. Schematic of crystal structure of rutile (left) and anatase TiO2 (right). (Reprinted with permission from
Reference [24], Nah, Y. C. et al., Chemphyschem, 2010, 11, 2698-2713. Copyright © Wiley-VCH).
Figure 2. Band gap energies and band positions of several semiconductors in contact with aqueous electrolyte
at pH 1. (Reprinted with permission from Reference [14], Gratzel, M. Nature 2001, 414, 338-344. Copyright ©
Nature Publishing Group).
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temperature anatase structure. Furthermore, the
photoactivity enhancement can be related to the
Fermi level of anatase (a band gap of 3.2 eV)
which is about 0.2 eV higher than that of rutile (a
band gap of 3.02 eV) [2,26]. However, rutile
TiO2 still has some advantages over anatase such
as higher chemical stability, higher refractive
index, and cheaper production cost, while also
performing comparably to anatase in
photocatalysis application [27]. Additionally,
monoclinic TiO2 (B) has a relatively open
structure with large voids and continuous
channels, which endows it with fast transport
paths for lithium intercalation as well as
excellent photoelectrical properties [15,28].
Comparatively, orthorhombic brookite TiO2 is
theoretically predicted to be a promising
dielectric with a static dielectric constant much
higher than anatase and rutile. This is most likely
because brookite is a metastable phase with a
very complicated and low symmetry structure
and as a result of the rapid transformation that
accompanies secondary phases such as anatase
or rutile [29]. More importantly, considerable
results have demonstrated that mixtures of
different phases, such as anatase/rutile [30],
anatase/brookite [21], and brookite/rutile [21]
(anatase/rutile in particular), possess
synergistically enhanced charge-carrier transfer,
which improves their performance in
photocatalytic applications [3].
The electronic properties of TiO2 have
been investigated by using a variety of
theoretical studies and experimental techniques
[2,15,24,31-36]. In brief, TiO2 is an n-type
semiconductor comprised of a conduction band
with an edge of low energy formed by the vacant
Ti4+
d bands, and a valence band with an upper
edge composed of O2-
filled pπ bands [24]. The
band-gap energy of indirect electron transition is
3.02 and 3.2 eV for rutile and anatase,
respectively. Apart from these definite band gap
values, the relative position of energy levels
compared to redox potentials of some species in
media are of great importance for numerous
functional applications of TiO2, which will be
discussed in the following section.
3. Applications
Since the discovery of hydrogen
evolution through the photoelectrochemical
splitting of water based on n-type TiO2
electrodes by Fujishima and Honda in 1972 [1].
semiconductor photocatalysts have attracted
great attention from both academic and industrial
communities. In particular, TiO2 nanomaterials
have found wide application in the fields of dye-
sensitized solar cells, hydrogen production, the
degradation of organic pollutants, sensors [37],
and photoinduced hydrophilicity [38], to name a
few. Herein, the application of TiO2
nanomaterials in solar cells, hydrogen
generation, and photocatalytic degradation of
organic pollutants are discussed in detail to
illustrate some promising alternatives for
renewable energy exploitation and environment
protection.
3.1. Dye-sensitized solar cells
Solar energy is clean, silent, and
endlessly abundant, making it one of the most
promising future energy resources. The direct
conversion of sunlight into electrical power by
solar cells is of particular interest because of its
many advantages over currently used electrical
power generation methods. For instance, solar-
based electricity is obtained without the exhaust
of green-house gases or nuclear waste by
products [12,13]. To date, silicon remains the
dominant material for solar cell systems [7].
However, the inherent deficiencies of silicon
solar cells, such as the complicated and energy
intensive fabrication process and inevitable use
of toxic chemicals, along with the heavy cell
weight and the high manufacturing costs, have
aroused a strong desire to develop cost-efficient,
versatile, and easily manufacturable alternatives
to the silicon status quo [35,39]. A dye-
sensitized solar cell (DSSC), also known as the
Gratzel cell, was first announced in 1991 with a
conversion yield of 7.1% [40]. This promising
electrochemical device relies on an operating
principle similar to the photo-synthesis reaction
of green plants. The DSSC is attractive for its
simple manufacturability, low cost, and use of
abundant and non-toxic materials [39,41]. In
addition, DSSCs performs better under
illumination from low light intensities and
diffuse light conditions than traditional silicon
solar cells, which makes them especially suitable
for low-power indoor applications such as laptop
computers and mobile phones. DSSCs can also
be fabricated on flexible substrates such as
plastic foils and metal sheets, making them
suitable for large scale, roll-to-roll type industrial
manufacturing processes [39]. In short, DSSCs
offer the possibility of designing photovoltaics
with tunable shape, color, and transparency as
well as feasible integration into large-scale
commercial products [35,42].
Typically, a DSSC consists of five
components: a conductive mechanical support, a
semiconductor film, a sensitizer, an electrolyte
and a counter electrode. Hence, the total
efficiency of DSSCs depends on the optimization
and compatibility of each of these constituents
[43]. In DSSCs, electricity is created at the
photoanode, which is formed of nanostructured
TiO2 materials covered with a monolayer of
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visible light absorbing dye [44]. Photon
absorption generates excitons in the dye via a
metal-to-ligand type transition, and the excited
electrons are efficiently injected into the TiO2
conduction band, which is significantly lower
than the excited state of the dye molecule. The
electrons diffuse through the TiO2 nanomaterial
into the current collector (e.g. transparent
conductive glasses), from which they can be
transferred into an external load [6,41,42,45].
Efficient collection of solar power begins with
sensitizing dyes, which should possess several
properties including stability, wide spectral
absorbance range, and a molecular structure
suitable for chemical bonding on TiO2 materials.
The most commonly used dyes are
organometallic ruthenium polypyridyl
complexes (e.g. N719 dye[ruthenium(2,2’-
bipyridyl-4,4’-dicarboxylate)2(NCS)2]), which
satisfy almost all of the above requirements
[42,46-48]. However, because ruthenium is a
rare metal, alternative dyes containing no metal
or only inexpensive metals have been developed
for DSSCs [49]. In addition to organometallic
dyes, quantum dots have also been explored as a
substitute for dyes because of the visible light
response ability of some kinds of quantum dots
(e.g. CdS, CdTe, and CdSe), nevertheless, the
conversion efficiency of these quantum dot solar
cells (QDSSCs) is still much lower than DSSCs,
mostly due to their instability when exposed to
visible light [50-52].
The photoanode in a DSSC is coupled
with the counter electrode (CE) through a layer
of redox-electrolyte separating the two
electrodes. The redox-electrolyte plays the
important role of transporting positive holes
generated during dye excitation to the counter
electrode, which is realized by alternating the
oxidation states of dye molecules on the
photoanode. The reduced species of the couple
(e.g. iodide or I-) contained in the redox
electrolyte donates electrons to the oxidized dye
to reduce it back to the ground state where it can
absorb photons again. The redox species renews
the oxidized state by itself (e.g. I- to I3
-) and
diffuses to the counter electrode, where electrons
returning from the external load reduce the redox
species back to its original state (e.g. I3- to I
-).
The reduced redox species then diffuses back to
the photoanode to start a new reaction circle
[6,41,42]. To date, the I-/I3
- redox couple, in
conjunction with ruthenium polypyridyl dyes,
has displayed the highest power conversion
efficiencies in DSSCs, and thus, this redox
couple is utilized in practically all liquid or
quasi-solid DSSC electrolytes [53]. I- ions are
typically introduced from an alkali metal salt
such as LiI or an imidazolium derivative called
ionic liquid electrolyte, and I3- emerges when I
-
react with I2 in the electrolyte. Besides the I-/I3
-
redox couple, the electrolyte commonly contains
other species such as 4-tert-butylpyridine (TBP),
which attach to sites on the TiO2 surface not
covered with dye in order to suppress dark
current in the cell. In addition, several low-
viscosity solvents are present in the electrolyte to
ensure that all constituents remain dissolved in
solution, for example, various nitriles (aceto-,
methoxypropio-, valero- or butyronitrile) or
mixtures of carbonates (ethylene/
propylenecarbonate) are found in many
electrolytes. A typical electrolyte consists of
methoxyproprionitrile (MPN) with 0.1~0.5 M
LiI, 0.1~0.5 M TBP, and 0.1 M I2 [53,54].
However, in the case of liquid electrolytes, cell
degradation is caused by solvent evaporation and
leakage over time. Accordingly, polymers can be
added to the ionic liquid to form the electrolyte
into quasi-solid gel (resembling solid state solar
cells) to minimize leakage [41,43,55,56].
As mentioned above, the counter
electrode returns charge from the external circuit
back to the cycling circuit in the cell. Thus, an
active catalyst layer coated on the counter
electrode surface is necessary to achieve
sufficient charge transfer. Platinum is the most
commonly used counter electrode material
because of its high catalytic activity for the
triiodide reduction reaction, chemical inertness,
and mechanical stability [6,41,42]. However, the
shortage and high price of platinum metal has
inspired the pursuit of other catalysts such as
activated carbons, carbon nanomaterials and p-
type conducting polymers [55,57,58].
Roughly, the overall operating cycle in
DSSCs can be described in Figure 3 and in the
following reactions [6,43]. Of note are
deleterious recombination processes within a
DSSC involving the recombination of
photoexcited electrons with electron acceptors,
such as oxidized ions and electron scavengers
located in the electrolyte or oxidized dye
molecules at the dye-electrolyte interface [59].
Several excellent reviews provide detailed
discussions of the kinetic and energetic behavior
of photoexcited electrons in DSSCs
[41,44,54,59,60].
S (TiO2) + h → S*
light absorption by the dye molecules (1)
S* → S
+ + e
- (TiO2) electron injection (2)
e- (TiO2) → e
- (current collector)
electron transport (3)
2S++3I(TiO2)→2S+ I3 (TiO2)
dye regeneration (4)
I3- (TiO2) → I3
- (CE) tri-iodide diffusion (5)
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I3-(CE) + 2e
-(CE) → 3I
-(CE)
iodide regeneration
(6)
3I (CE) → 3I (TiO2) iodide diffusion (7)
DSSC performance can be effectively
characterized using the following parameters:
maximum short-circuit current (Jsc), open-circuit
photovoltage (Voc), fill factor (FF), and overall
solar-to-electrical energy conversion efficiency
(η) [54]. For a solar cell, η can be determined
using equation (8), where Pin is the intensity of
the incident light. The FF can assume values
between 0 and 1 and is defined by the ratio of the
maximum power (Pmax) generated by the solar
cell per unit area to the Voc and Jsc according to
equation (9). Pmax is obtained from the product of
the photocurrent and photovoltage at the voltage
where the power output of the cell is maximal
[48]. Another fundamental evaluation of solar
cell performance is “external quantum
efficiency”, which is normally called the incident
photon to current conversion efficiency (IPCE).
IPCE values provide useful information about
the monochromatic quantum efficiencies of a
solar cell since they are defined by the
photocurrent density generated in an external
circuit under monochromatic illumination of the
cell divided by the photon flux that hits the cell.
IPCEs can be calculated from equation (10),
where e is the elementary charge [60].
(8)
(9)
(10)
As the core of DSSCs, TiO2
nanomaterials not only need to transport the
photoexcited electrons into current collectors as
quickly as possible, but also need to serve as
effective anchors for sensitized dyes to adsorb to.
To date, an approximately 10-µm thick film with
a 3D network composed of interconnected
spherical TiO2 nanoparticles (NPs), has obtained
an overall power conversion efficiency of over
11% [61]. However, in this disordered network
numerous grain boundaries are believed to
restrict the excited electron transport. In this
regard, considerable research is devoted to the
fabrication of various nanostructured TiO2
materials, such nanospheres, nanotubes,
nanorods, nanowires and combined structures in
order to alleviate this blockage of current flow.
3.2. Photocatalytic hydrogen generation
In the face of fossil-fuel shortages and
worsening environmental pollution, hydrogen-
based energy systems have attracted extensive
attention. Hydrogen is a clean energy because
chemical energy stored in the H-H bond is easily
released when the molecule reacts with oxygen
to yield water as a byproduct. In order to realize
a hydrogen economy, future energy devices
capable of generating hydrogen sustainably will
be necessary. In addition to holding promise as
an energy source, hydrogen is also a versatile
energy carrier that is currently produced from a
variety of primary sources such as natural gas,
heavy oil, methanol, biomass, wastes, coal, solar,
wind and nuclear power. Among these sources,
hydrogen production using solar irradiation
attracts the greatest attention because of its
potential to utilize solar energy to split water in
the form of heat, light, or electricity. The direct
use of sulnight provides the most efficient route
of hydrogen production because it avoids the
inefficiencies ascribed to thermal transformation
or electrolysis following the conversion of solar
Figure 3. The operating principle of DSSCs based on TiO2 nanomaterials.
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energy to electricity [12,13,62]. Water
decomposition using sunlight in the presence of
semiconductor photocatalysts has attracted
intense research interest since the pioneering
work on a photo-electrochemical cell
demonstrated by Honda and Fujishima in 1972
[1].
Thermodynamically, the overall water-
splitting reaction is an uphill reaction with a
significantly positive change in Gibbs free
energy (ΔG0=+237.2 kJ/mol, 1.23 eV per
electron). The absorbed photon energy needs to
overcome this large Gibbs free energy for water
splitting [62]. Since the electrochemical
decomposition of water to hydrogen and oxygen
is a two-electron stepwise process, it is feasible
to use photocatalytic surface reactions which rely
on solar energy to generate electrons and holes
that can respectively reduce and oxidize the
water molecules adsorbed on photocatalysts.
However, the applied semiconductor
photocatalyst should possess band gap energy
(Eg) greater than 1.23 eV to overcome the Gibbs
free energy for hydrogen generation. In addition,
the band position of the photocatalyst is also
important. That is, for easy electron-hole pair
transfer, the semiconductor conduction band
should be located at a potential more negative
than the reduction potential of water, while its
valence band should be located at more positive
position than the oxidation potential of water
[1,62-66].
It has been published that some metal
oxides, (for instance, TiO2, SrTiO2, ZrO2,
BaTi4O9, and CeO2) possess reasonable activities
for water splitting into H2 and O2 in the
stoichiometric ratio under ultraviolet (UV) or
visible light irradiation [62]. TiO2 has been
widely used in photocatalysis, owing to its
favorable band-gap energy (3.2 eV in anatase)
and high stability in aqueous solution under UV
irradiation. The photocatalytic process of TiO2 is
initiated by the absorption of a photon with
energy matching or greater than the forbidden
band gap energy of the semiconductor.
Subsequently, electron-hole pairs are generated
following light irradiation. Photoexcited
electrons migrate to the conduction band where
they reduce H+ to H2 (see reactions 11 and 12,
Figure 4), and holes on the surface of the
semiconductor decompose H2O to O2 (see
reaction 13 and 14, Figure 4) [4,62,64,66,67].
(11)
(12)
(13)
(14)
However, in order to be excited by
visible light irradiation, the band gap energy of
the semiconductor should be less than 3.0 eV.
Consequently, although the large band gap of
TiO2 (3.2eV for anatase) is crucial for many
applications, it hampers the efficient utilization
of visible light (only about 7% of solar energy in
the spectral range has a wavelength less than 400
nm). Therefore, remarkable efforts have focused
on modifying the electronic properties of TiO2
through shortening the optical band gap, doping
the material, or suitable modification of the TiO2
surface (sensitization or junction formation) to
create a valuable visible-light response
[3,4,24,62,64]. In a simple aqueous solution,
pure TiO2 cannot split water into H2 and O2
because of the fast, undesired electron-hole
recombination reaction, which is
thermodynamically favored. To counter this
deleterious reaction, so-called sacrificial reagents
are added to the water. Their main function is to
separate the photoexcited electrons and holes
that are available for reversible reaction by
playing the role of electron donors or electron-
acceptor scavengers. For photocatalytic
hydrogen generation, compounds such as
methanol, ethanol, EDTA, Na2S, NaCO3,
Figure 4. Overview principle of TiO2-based photocatalytic water splitting for hydrogen generation.
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Na2SO4, or ions such as I-, IO3
-, CN
-, and Fe
3+
can be used as sacrificial reagents [67].
The efficiency of photocatalytic
hydrogen generation from water splitting can be
measured directly from the amount of generated
hydrogen gas or indirectly from the electrons
transferred from semiconductor to water within a
certain time period under light irradiation.
Commonly, the rate of gas (O2 and H2) evolution
with units such as µmol·h-1
and µmol·h-1
·g-1
is
used to make measurable comparisons between
different photocatalysts under similar
experimental conditions. Another measure of
photocatalytic activity, the (apparent) quantum
yield, is an extension of the overall quantum
yield in a homogeneous photochemical system,
and these parameters can be calculated by
equation 15 and 16, respectively. In addition to
quantum yield, the solar energy conversion
efficiency that is usually used for evaluation of
solar cells is also sometimes applied and
estimated by equation 17 [66].
(15)
(16)
(17)
Significantly, TiO2 was the first
discovered photocatalyst for water splitting
under UV irradiation, where the corresponding
quantum efficiency was approximately 10% in
the case of Fe3+
doping [1]. Furthermore,
colloidal TiO2 combined with Pt and RuO2
particles generated H2 and O2 in stoichiometric
proportions from water with a high quantum
yield of 30±10% under UV irradiation [66].
Other published results about hydrogen
generation from water splitting via solar
irradiation based on TiO2 nanomaterials will be
shown in later sections.
3.3. Photocatalytic degradation of organic
pollutants
As industrialization and population
growth have spiked in recent years, the
environmental contamination caused by
wastewater pollutants has become a worldwide
problem. Wastewaters from various industries
pose a serious environmental threat because they
contain organic pollutants that are toxic to
microorganisms, aquatic life, and human beings
[17,68]. In the past decades, conventional
biological and physical treatment methods, such
as adsorption, ultrafiltration, and coagulation
have served as popular techniques to remove the
organic pollutants from various wastewaters
[17]. However, novel techniques are actively
being pursued for the decontamination of many
artificial or anthropogenic organic pollutants,
especially those with high toxicity but very low
concentration. These techniques should be able
to chemically transform the organic pollutants
into non-hazardous compounds efficiently and
rapidly. Semiconductor nanomaterials hold
promise for inexpensive and environmentally
friendly decontamination systems, in which the
correlated chemical reagents, energy source and
catalysts could be abundant, cheap, and nontoxic,
while producing no secondary pollution
byproducts [68,69].
Recently, considerable studies have
been devoted to the utilization of photocatalysis
in the degradation of organic pollutants from
wastewaters, particularly due to its ability to
completely mineralize the toxic organic
chemicals into non-toxic inorganic minerals
[5,17,63,68,69]. Photocatalysis is a photoinduced
reaction that can be improved by using a catalyst.
Semiconductors (such as TiO2, ZnO, Fe2O3,
CdS, and ZnS) can serve as sensitizers for light-
induced redox-reactions because of their unique
electronic structure, which is characterized by a
filled valence band, and an empty conduction
band [5]. Under irradiation, the valence band
electrons are excited to the conduction band
leaving a hole behind. The resulting electron–
hole pairs can recombine or react separately with
other molecules. The holes may migrate to the
surface and react either with electron donors in
the solution or with hydroxide ions to produce
powerful oxidizing species like hydroxyl or
superoxide radicals. Meanwhile, the conduction
band electrons can reduce an electron acceptor.
Consequently, semiconductor materials can act
as either an electron donor or an electron
acceptor for molecules in the surrounding
medium, depending on the charge transfer to the
adsorbed species [63,70]. Compared to other
semiconductors, TiO2 is the most widely used
semiconductor catalyst in photoinduced
processes because it is chemically and
biologically inert, photostable, relatively easy to
manufacture and utilize, able to efficiently
catalyze reactions, inexpensive, and nontoxic,
though it has the disadvantage of activation only
by ultraviolet (UV) light and not visible
irradiation [5,15 68,71].
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In TiO2, the photocatalytic reactions are
principally activated by absorption of a photon
with sufficient energy (equal to or higher than
the band gap energy (Eg) of TiO2) to produce an
electron-hole pair. The photoexcited electrons
can react with O2, reducing it to superoxide
radical anion, O2−•
, or reduce the organic
chemicals, which act as the electron acceptors
adsorbed on the TiO2 surface or dissolved in
water. Meanwhile, the photogenerated holes can
oxidize organic molecules to form oxidized
products or react with OH− or H2O to form OH
•
radicals. The resulting •OH radical, a very strong
oxidizing agent (standard redox potential
+2.8V), can oxidize most organic chemicals to
mineralized products [72,73]. Accordingly, the
relevant reactions on the TiO2 surface resulting
in the degradation of organic pollutants can be
expressed in the following reactions as well as in
Figure 5 [17,68].
(18)
(19)
(20)
(21)
(22)
(23)
(24)
(25)
However, for organic chemical
degradation, TiO2 materials still suffer the
limitation of activation solely by UV light (λ<
390 nm). Therefore, several remediation
strategies have been employed, for instance, the
bulk-doping approach is frequently used to
develop TiO2-based visible-light-response
photocatalytic systems [3,68].
4. Synthesis methods
Recently, TiO2 nanomaterials, including
nanoparticles, nanorods, nanowires, and
nanotubes have been widely investigated, and
various synthesis methods, including, sol-gel
[74,75], hydrothermal/solvothermal [76],
physical/chemical vapor deposition [31],
electrochemical anodization, and direct chemical
oxidation [15,77], have been successfully applied
in fabricating them. In this section, some popular
methods are discussed with a specific focus on
the properties of TiO2 nanostructures attained
from each technique.
4.1. Sol-Gel
TiO2 nanomaterials have been
frequently synthesized via the sol-gel method, a
widely used method in making various ceramic
materials. In a typical sol-gel process, a colloidal
suspension or sol is formed from the hydrolysis
and polymerization reactions of the precursors,
which are usually inorganic metal salts or metal
organic compounds such as metal alkoxides.
Subsequent complete polymerization and loss of
solvent induces a phase conversion from the
liquid sol into a solid gel phase. Furthermore,
this technique allows for the production of thin
films on a variety of substrates by spin-coating
or dip-coating. Hence, nanomaterials with novel
properties can be obtained via the sol-gel process
under optimized conditions. TiO2 synthesis using
the sol-gel method occurs from the hydrolysis of
a titanium precursor, and this process is normally
performed via an acid-catalyzed hydrolysis step
of titanium (IV) alkoxide or halide precursor
followed by condensation. Properties of the
resulting nanomaterials, such as the degree of
crystallinity, morphology, structure size, surface
area, and degree of agglomeration, depend on the
reaction conditions, including the temperature,
Figure 5. Simplified principle of TiO2-based photocatalysis of organic pollutants.
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evaporation rate, drying conditions, and post-
treatment. The rates of hydrolysis and
condensation are largely affected by the type of
precursors, the pH, and the mixture of solvents
[4,15,74]. Among various methods, the sol–gel
method is one of the most widely used for TiO2
nanostructure preparation, due to its advantages
of relatively low cost, flexibility of substrate
(several sizes and shapes are possible), and the
wide range of nanostructures that can be
obtained (from nanoparticles, nanorods or
nanoporous films, to nanowires) [11].
4.2. Hydrothermal/solvothermal
The hydrothermal/solvothermal method
has also been widely used to prepare TiO2
nanomaterials. In this case, the synthesis reaction
is normally carried out in aqueous/organic
solutions within steel pressure vessels called
autoclaves under controlled temperature. The
temperature can be raised above the boiling point
of the water/organic solvent, reaching the
pressure of vapor saturation. The temperature
and the amount of solution added to the
autoclave largely determine the internal pressure
produced. The solvothermal method is almost
identical to hydrothermal method except that the
solvents used for solvothermal synthesis are non-
aqueous, while water is the primary
hydrothermal solvent. Consequently, the
temperature in solvothermal process can be
elevated much higher than that in hydrothermal
method because a variety of organic solvents
with high boiling points can be adopted.
Normally, the solvothermal method possesses
better control than hydrothermal methods over
the size, structure, size distribution, and degree
of crystallinity of TiO2 nanoparticles [4].
In comparison with other methods,
hydrothermal/solvothermal synthesis is a facile
route to prepare a highly-crystalline oxide under
moderate reaction conditions, such as low
temperature (in general < 2500C) and short
reaction time. Meanwhile, it provides an
effective reaction environment for the formation
of nanocrystalline TiO2 with high purity, good
dispersion and well-controlled crystallinity. With
this method, the sintering process, which results
in a transformation from the amorphous phase to
the crystalline phase, can be avoided. Similarly,
by tuning the hydrothermal conditions (such as
temperature, pH, reactant concentration and
molar ratio, and additives), crystalline products
with different composition, size and morphology
can be achieved.
4.3. Electrochemical anodization
The controlled oxidation of titanium
metal under electrochemical anodization
provides another means of TiO2 nanomaterial
production. This technique is specifically
amenable to the synthesis of TiO2 nanotubes
from titanium foil, which has been extensively
studied after pioneering work in 2001 by Gong
and co-workers, in which they reported the
formation of nanotubes up to 0.5 mm length by
electrochemical anodization of titanium foil in
HF aqueous electrolyte [78]. The anodization of
titanium is generally conducted in a two-
electrode electrochemical cell at a constant
potential in aqueous or organic electrolyte
containing F− with platinum foil as a cathode.
Normally, the anodization process can be
roughly divided into three stages: (1) the
electrochemical oxidization of the titanium
surface results in the formation of an initial TiO2
barrier layer, corresponding to the first current
drop, (2) TiO2 is chemically etched by F- to form
TiF62−
, resulting in nanotube formation that leads
to a current increase, and (3) the growth of
nanotubes relates to a slow current decrease.
Briefly, the nanotube growth is determined by
the equilibrium between anodic oxidation and
chemical dissolution. The anodic oxidation rate
is mainly controlled by the anodic potential,
while the chemical dissolution rate is greatly
governed by the electrolyte acidity and F−
concentration [79-82].
Compared to other methods, anodic
oxidation is a relatively simple technique that
can be easily automated for the preparation of
TiO2 nanotube arrays with highly ordered
structure and controllable size, which is mainly
determined by the applied anodic potential,
electrolyte composition, temperature, and time.
Moreover, under optimized conditions, it also
feasible to prepare other TiO2 nanoarchitectures,
such as nanorods and mesoporous structures [4].
4.4. Physical/chemical vapor deposition
Vapor deposition describes a procedure
in which materials in a vapor state are condensed
to form a solid phase material, and the whole
process is usually conducted within a vacuum
chamber. If no chemical reaction occurs, this
process is called physical vapor deposition
(PVD), otherwise it is called chemical vapor
deposition (CVD) [4,15]. In PVD, materials are
evaporated and subsequently condensed to form
a solid material. The primary PVD methods
include thermal deposition, ion plating, ion
implantation, sputtering, laser vaporization, and
laser surface alloying [4,15]. However, in CVD
processes, thermal energy heats the gases in the
coating chamber to induce a deposition reaction.
Typical CVD approaches include electrostatic
spray hydrolysis, diffusion flame pyrolysis,
thermal plasma pyrolysis, ultrasonic spray
pyrolysis, laser-induced pyrolysis, atmospheric
pressure, and ultrasonic-assisted hydrolysis
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[4,15,31,83]. Several TiO2 nanostructures
prepared though PVD or CVD have been
reported. For example, TiO2 nanoparticles with
sizes below 10 nm were prepared by pyrolysis of
TTIP via a CVD process in a mixed
helium/oxygen atmosphere [84]. TiO2 nanorods
were grown on a Si substrate using TTIP as the
precursor by metal organic CVD (MOCVD)
[85], and aligned TiO2 nanowire arrays were
fabricated onto Si wafers by a simple thermal
deposition (PVD) method [86].
4.5. Chemical (template) deposition
Templated synthesis of nanostructured
materials has become extremely attractive during
the last decade. This method utilizes the
morphological properties of known and
characterized templates in order to assemble
materials with a similar morphology by reactive
deposition or dissolution methods. By adjusting
the morphology of the template material, it is
possible to prepare numerous new materials with
a regular and controllable morphology on the
nano- and micro scale. One of the most
commonly used templates, an alumina
membrane having uniform and parallel porous
structures, is prepared by anodic oxidation of
aluminum sheets in solutions of sulfuric, oxalic,
or phosphoric acids [11]. Many TiO2
nanostructures have been achieved through
template assisted deposition processes. For
instance, large-area free-standing arrays of TiO2
nanorods and nanotubes were selectively
synthesized on transparent conducting indium tin
oxide (ITO) substrates using anodic alumina
(AAO) thin film templates [87], highly oriented
TiO2 nanotubes have also been fabricated using
ZnO nanorod template through liquid reactive
deposition on the ITO substrates [88].
The template approach is suitable for
preparing nanoarrays of materials that are
difficult to produce by other solution techniques.
Nevertheless, in most cases, the template
material is sacrificial and needs to be removed
after synthesis, leading to an increase in the cost
of materials. As in the case of all surface-
finishing techniques, it is important to maintain a
high level of surface cleanliness to ensure good
adhesion between the template and the surface
coating.
It is worth noting that in practice, novel
morphologies are often obtained using a
combination of two or three methods. For
example, TiO2 nanoparticles and films can be
obtained by treating titanium isopropoxide with
water, followed by hydrothermal post-treatment
[89,90], or electrochemical oxidation together
with sol treatment [91].
5. TiO2 nanoarchitectures
A large internal surface area is the
foremost requirement for the photoinduced
reactions in DSSCs, water splitting, and organic
chemical photocatalysis. Nanomaterials can
satisfy this criterion because the specific surface
area of nano-scale morphologies may be more
than 1000 times greater than bulk materials. In
an effort to enhance photocatalytic efficiency,
various architectures of nanostructured TiO2
have been investigated including nanoparticles,
nanorods, nanowires, and nanotubes. In this
section, a series of nanostructured TiO2
architectures are discussed. An effort is made to
summarize typical synthesis procedures for each
structure, and representative transmission or
scanning electron microscopy images are
provided to give the reader an appreciation for
the variety of nanoarchitectures currently
available.
5.1. TiO2 nanoparticles
In the past decade, the number of
literature articles about nanostructured TiO2
materials has increased considerably.
Particularly, nanoparticle TiO2 thin films have
attracted great attention, partially due to the high
surface area to volume ratio afforded by
numerous surface-active sites, the direct
availability of porous structures with assembled
nanoparticles for electrolyte infiltration, and the
simplicity of manufacturing films using
chemically based solution methods [69]. One
commercial form of TiO2 nanoparticles, Degussa
P25, is a standard industrial photocatalyst
prepared by the hydrolysis of TiC14 in pure
water or a 1:1 EtOH-H2O solution. The relatively
short residence time necessary for the conversion
of TiCl4 to TiO2 gives a product which has a
high surface area (~50 m2g
-1) and phase mixture
of approximately 4:1 anatase to rutile [2,63].
However, for a specific photoinduced reaction
system, it is desirable to optimize the particle
size and crystal phase as well as select a suitable
synthesis method. This has prompted the
development of several experimental procedures
for preparing TiO2 nanoparticles as well as
theoretical strategies for structure prediction
[25,34,92-96].
TiO2 nanoparticles have been
extensively prepared by the sol-gel method.
Numerous research efforts have shown that
various parameters related to the sol-gel process
have a great impact on both the TiO2
nanoparticle phase and average particle sizes
[32,36,89,90,97-101]. For example, it was found
that decreasing the Ti/H2O molar ratio resulted in
both decreasing brookite content and anatase
particle size in sol-gel synthesized TiO2
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nanoparticles [89]. In addition, hydrothermal
aging, a common post-treatment for sol-gel
synthesis, has several tunable variables such as
pH [90], dialysis [89], and temperature [89],
which have been proven to influence particle
growth and relative phase stability. However, the
products of hydrothermal aging depend most
strongly upon the initial phase composition and
average particle sizes after sol-gel synthesis,
which are most sensitive to the pH employed
during sol-gel synthesis. Therefore, substantial
control over the brookite content, particle size,
and particle growth mechanism can be achieved
simply by changing the pH of the sol and gel
[90]. Furthermore, the growth kinetics of TiO2
nanoparticles synthesized from sol-gel processes
has been investigated. It was demonstrated that
the aggregation kinetics of TiO2 nanoparticles
was influenced by the ionic strength and pH of
suspensions and the nature of the different cation
valences present in the electrolyte [97,102], yet,
the average particle radius increased linearly
with time in agreement with the Lifshitz-
Slyozov-Wagner model for coarsening, where
the rate constant for coarsening elevated with
temperature [98].
It is worth noting that although the
conventional sol-gel process is capable of large-
scale production, it is incapable of producing
TiO2 nanoparticles with controlled shape and
size distributions. Accordingly, a novel two-step
sol-gel process has been developed to synthesize
uniform TiO2 nanoparticles. In this method,
oxide particles are grown via two steps: (1) the
formation of a hydroxide gel and (2) nucleation
and growth of oxide particles. This process is
distinguishable from the conventional sol-gel
method, in which the gel phase develops first
and is converted to a sol phase. The two-step sol-
gel process is based on the idea of using a highly
condensed precursor metal hydroxide gel as a
protective matrix against coagulation of the
growing particles as well as a reservoir of metal
ions. With this strategy, TiO2 nanoparticles of
uniform size distribution and various shapes can
be obtained, and anatase nanoparticles
synthesized this way were shown to possess high
crystallinity, size uniformity, negligible surface
defects, and negligible residual organic
compounds, all of which are beneficial properties
for photovoltaic applications (see Figure 6).
When organized into a DSSC sensitized by N719
dye, the photoanode exhibited a much higher
efficiency (6.72%) than that of the P25 TiO2
(4.0%) because of enhanced dye adsorption and
charge transport [75].
It is well documented that TiO2
nanoparticles can also be prepared by
hydrothermal procedures. The influence of
various hydrothermal parameters on the
formation, phase, morphology, and grain size of
products have been investigated and discussed in
many published reports [36,76,103-109]. In
general, the particle growth kinetics under
hydrothermal conditions is believed to be
determined by coarsening and aggregation–
recrystallization processes, allowing control over
the average nanoparticle size [110,111]. For
instance, pure phase TiO2 nanoparticles in
anatase, rutile and brookite structures as well as
their mixtures were fabricated using amorphous
TiO2, titanate alkoxide, or halide salts as
precursors [104,105,110,111]. Particles of
different phases could be achieved by
hydrothermal treatment at elevated temperatures
and suitable pH values with the appropriate
reactants [110,111]. Anatase nanoparticles were
obtained using acetic acid, while pure rutile and
brookite nanoparticles were obtained with
hydrochloric acid at a different concentration
Figure 6. TEM (a, c) and HR-TEM (b, d) images of the TiO2 nanoparticles prepared by sol-gel process.
(Reprinted with permission from Reference [75] (a) and (b): Lee, S. et al., Chem. Mater. 2010, 22, 1958-1965.
Reference [89] (c) and (d): Isley, S. L. et al., J. Phys. Chem. B 2006, 110, 15134-15139. Copyright ©
American Chemical Society).
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[111], rutile precipitation at low pH and anatase
growth at high pH were promoted [110]. It was
proposed that anatase formation was dominated
by surface energy effects, while the formation of
rutile and brookite acted in a dissolution–
precipitation mechanism, where chains of six
fold-coordinated titanium complexes gathered
into different crystal structures [111].
Particularly, anatase TiO2 particles with specific
exposed crystal faces could be prepared by
hydrothermal treatment of peroxo titanic acid
(PTA) solution with polyvinyl alcohol as a
shape-control reagent. TiO2 particles prepared
from a PTA solution of pH 7 had (101) and (001)
exposed crystal faces, which showed higher
photocatalytic activity for acetaldehyde
decomposition than commercial spherical TiO2
particles, and the shape of TiO2 particles
changed with the time of hydrothermal treatment
(see Figure7) [106].
Besides sol-gel and hydrothermal
methods, other common synthesis techniques
have been applied to prepare TiO2 nanoparticle
films. For instance, recently, a novel and facile
method termed phase-separation-induced self-
assembly was developed to prepare TiO2
nanodots with tunable size (28~93nm) and
density (4.05 × 1010
~ 0.45 × 1010
cm−2
) on a
substrate (see Figure 8), and it was found that
smaller nanodots exhibited a pronounced
hydrophilic characteristic with improved
wettability due to an increase in solid–liquid
area. This method induced convective flow in a
spin-coated titanium tetrabutoxide
(TBOT)/polyvinyl pyrrolidone (PVP)/ethanol
liquid film through the Marangoni effect.
Figure 7. TEM images of TiO2 nanoparticles prepared by hydrothermal process in different
conditions.(Reprinted with permission from Reference [106] , Murakami, N. et al., J. Phys. Chem. C 2009,
113, 3062-3069. Copyright © American Chemical Society).
Figure 8. SEM images of TiO2 nanodots with different morphologies prepared on silicon substrates (bar
represents 200 nm). (Reprinted with permission from Reference [112] , Luo, M. et al., Nanotechnology 2009,
20. Copyright © IOP Publishing).
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Subsequent calcination transformed TBOT into
crystalline TiO2 nanodots on the substrate [112].
Another femtosecond and nanosecond pulsed
laser deposition (PLD) technique at different
laser wavelengths has been widely used to
construct nanoparticle-assembled TiO2 films.
This technique allows for control of the
dimensions and crystalline phase of
nanoparticles by varying the laser parameters,
and the deposition conditions and suitable for
depositing TiO2 films at a relative high rate and
low cost [113-115].
The most efficient (~11% PCE) TiO2
electrodes in DSSCs use 10-μm-thick TiO2
nanoparticle films consisting of an
interconnected network of nanometer-sized
crystals ~20 nm in diameter [61]. It has been
confirmed that this type of structure imparts
sufficient surface area to adsorb a large amount
of dye molecules, leading to efficient light
absorption and charge formation [116-118]. The
effects of particle size on the overall
performance of DSSCs have been investigated.
Larger particle-based films have larger contact
points between sintered colloidal particles and at
the interface between the particles and the
underlying substrate. The relatively small
number of grain boundaries beneficially
improves charge transport, though the small
surface area deleteriously limits dye adsoprtion.
Smaller particle-based films provide a larger
surface area together with a greater number of
contact points between sintered colloidal
particles and between particles and the
underlying substrate. The large surface area
allows for greater dye adsorption, but the
increased number of contact points reduces
charge mobility by increasing electron trapping
events [117,119,120]. The mechanisms of
particle size effect on electron injection
efficiency in ruthenium dye-sensitized TiO2
nanoparticle films were investigated by using
time-resolved fluorescence spectroscopy and
femtosecond (transmittance and diffuse
reflectance) transient absorption spectroscopy. It
was found that small diameter systems gave high
electron injection yields (around 90%), whereas
larger diameter systems gave low efficiencies
(from 35 to 70%). The dye loading of the larger
diameter systems was greater than that of small
diameter systems because more than a
monolayer of dye adsorbed to the larger
particles, which resulted in low efficiencies in
the larger diameter systems. Importantly, the
comprehensive and systematic time-resolved
spectroscopic data pointed out that the dye
aggregates formed in spaces between TiO2
nanoparticles were the crucial factor for reducing
electron injection efficiency rather than the size-
dependent surface nature of TiO2. In brief, for
DSSC application, dye aggregation must be
carefully treated and avoided in order to achieve
high performance, especially when using larger
semiconductor nanoparticles, which often work
as a light-scattering material [120].
5.2. Mesoporous TiO2
Mesoporous materials containing
nanoparticles distributed throughout structures of
adjustable pore size and high specific surface
area have attracted considerable interest during
the past decade. In particular, mesoporous TiO2
materials with tailored pore size, high specific
surface area, and well-defined crystalline
configuration have potential applications in solar
cells, photocatalysis, and water splitting.
Importantly, the pore size and specific surface
area of mesoporous TiO2 materials greatly
influences their physical properties related to
photocatalytic activity. For instance, in catalytic
applications, a tunable pore size can facilitate the
diffusion rate of reactants toward adsorption
sites, while a high surface area can maximize the
interface between the reactants and the catalyst
surface. For DSSCs, mesoporous TiO2 can
enhance light harvesting within the electrodes
without sacrificing the accessible surface for dye
loading [121-123].
When prepared via sol-gel methods, the
pore size of mesoporous TiO2 can be tuned by
addition of a structure-directing agent with
different hydrophobic chain lengths (e.g.,
triblock copolymer, Pluronic F127) or by
addition of a swelling agent (e.g., mesytilene),
which can be removed either by calcination in air
or by reaction with a solvent when the synthesis
procedure is completed. However, the specific
surface area is determined mostly by the
hydrolysis ratio. Usually, a high water/alkoxide
ratio enables a more complete hydrolysis of the
titanate alkoxide precursor, supporting
nucleation instead of particle growth and leading
to small particles with high specific surface areas
[121,124,125]. The sol-gel process using organic
surfactants as assisting templates represents the
most widely used route for the synthesis of
mesoporous TiO2 and involves a complicated
mechanism called evaporation-induced self-
assembly (EISA). The EISA process produces an
ideal grid-like morphology consisting of a
continuous, ordered network of anatase TiO2
with a high surface area by condensation of a
titanium precursor around self-organized organic
templates in a gel phase, followed by removal of
the templates. With this method, electrodes can
be prepared directly on Si or FTO substrates.
Furthermore, the process is highly scalable
because it can be performed at low temperature
without any expensive or complicated equipment
[121,122,126,127]. For example, ordered
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mesoporous TiO2 nanocrystalline films prepared
via layer-by-layer deposition with Pluronic P123
as a template (see Figure 9) exhibited enhanced
solar conversion efficiency with a remarkable
enhancement of the short circuit photocurrent
due to the large surface area accessible to both
the dye and the electrolyte [125]. Pluronic P123
was also employed in another work as a template
to attain optimized pore morphology, high
crystallinity, and stable mesoporous TiO2 films.
These films were then modified by adding P25
nanoparticles that acted as scattering centers and
functioned as active binders to prevent the
formation of microcracks. The combination of
increased surface area, light-scattering particles,
and high crystallinity of the mesoporous films
successfully enhanced the performance of
DSSCs [124]. In another approach for the
synthesis of mesoporous TiO2, a simple method
without organic structure-directing agents or
pretreated substrates was developed to produce
anatase TiO2 through a reaction-limited
aggregation in boric acid solution. The
hydrolysis rate along with the motion of the TiO2
nanoparticles could be controlled by adjusting
the concentration of boric acid. A solar cell
based on this mesoporous TiO2 possessed high
surface area, which increased dye loading and,
subsequently, improved both photocurrent and
cell efficiency [128].
Mesoporous TiO2 can also be obtained
though other procedures. For example, porous
anatase TiO2 films on stainless steel substrates
were fabricated with Ti(OC4H9)4 as a precursor
via a hydrothermal process. The degree of
crystallinity and porosity of the films were
determined by the time and temperature of the
hydrothermal reaction, and the resultant TiO2
films showed high photocatalytic activity
towards degradation of gaseous formaldehyde
[129]. Furthermore, a thick self-organized
mesoporous TiO2 layer was grown on titanium
foil by anodization in an organic/K2HPO4
electrolyte followed by chemical etching of the
structure. This material, which consisted of a
strongly interlinked network of nanosize TiO2,
provided a considerably higher specific surface
area than TiO2 nanotubes, and displayed
enhanced conversion efficiency as an electrode
in DSSCs after undergoing a TiCl4 hydrolysis
treatment (see Figure 10) [130].
5.3. TiO2 microspheres
It is well known that, in spite of the
large surface area of the TiO2 nanoparticle-based
film, it often suffers from having a disordered
morphology with many grain boundaries, which
gives rise to interfacial interferences to electron
transport. Moreover, when using typical 10–20
nm-sized TiO2 nanoparticles, the sizes are so
much smaller than the wavelength of visible
light that the film is transparent with little light
scattering, which leads to poor light harvesting
[131]. Accordingly, in order to satisfy the
requirements for fast electron transport and high
surface area along with enhanced light-scattering
in thin-film photoelectrodes, spherical (or bead-
like) TiO2 with a submicrometer-sized diameter
is under intense study for its relatively high
refractive index and particle sizes, which are
comparable to the wavelengths of optical light
[132-134]. Sub-micrometer-sized TiO2 spheres
have often been prepared using sol–gel methods
by controlling the hydrolysis and condensation
reactions, followed by subsequent calcination to
form crystallized structures. Monodispersed
spherical TiO2 structures can be successfully
obtained by this procedure, but their low surface
Figure 9. SEM images of mesoporous TiO2 films. (Reprinted with permission from Reference[125] (a)~(b):
Zukalova, M. et al., Nano Lett. 2005, 5, 1789-1792. Reference [126](c)~(f): Hartmann, P. et al., Acs Nano
2010, 4, 3147-3154. Copyright © American Chemical Society).
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area and poor pore structure impede widespread
application [135,136].
To overcome the low surface area of
microspheres as a result of smooth surfaces,
some post-synthesis treatments (commonly
solvothermal in nature) have been developed to
produce special morphologies, including
mesoporous microspheres and hollow
microspheres [137]. For instance, crystalline
mesoporous TiO2 beads with surface areas up to
108.0 m2g
-1 were synthesized through a facile
combination of sol-gel and solvothermal
processes (see Figure 11). Due to their
submicrometer-sized particle diameters and high
specific surface areas, the mesoporous TiO2
beads can enhance light harvesting in electrodes
without sacrificing the amount of accessible
surface area for dye loading, thereby increasing
the conversion efficiency of DSSCs created with
post-synthesis treated NPs (7.20%) compared to
P25 nanoparticles (5.66%) [132]. Similarly, after
the synthesis of sub-micrometer-sized TiO2
spheres by sol–gel methods, a specially designed
solvothermal treatment was employed instead of
the conventional calcination process to develop
crystallized nanoporous TiO2 spheres with
ultrahigh surface area and well-developed
nanopore structure, which yielded an enhanced
performance in DSSCs [135]. Recently,
mesoporous TiO2 beads were synthesized using a
sol-gel process in the presence of
hexadecylamine, following by solvothermal
treatment in an ethanol-water mixture containing
ammonia. When serving as the electrodes of
Figure 10. Cross-sectional SEM images of TiO2 mesosponge formed on Ti by anodization in 10 wt%
K2HPO4 in glycerol at 180 ± 1 0C as-formed (a, b), after etching in 30 wt% H2O2 for 1 h (c, d), and after
treatment of the TiO2 mesosponge in 0.1 M TiCl4 (e, f). (Reprinted with permission from Reference [130],
Kim, D. et al., Electrochem. Commun. 2010, 12, 574-578. Copyright © Elsevier).
Figure 11. SEM images of the precursor material (a, b), and the mesoporous TiO2 beads obtained after a
solvothermal process with different amounts of ammonia (c, d, e, f). (Reprinted with permission from
Reference [132], Chen, D. H. et al., Adv. Mater. 2009, 21, 2206. Copyright © Wiley-VCH).
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© 2011 Simplex Academic Publishers. All rights reserved.
DSSCs, the as-prepared mesoporous TiO2 beads
demonstrated longer electron diffusion lengths
and extended electron lifetimes over Degussa
P25 electrodes due to the well interconnected,
densely packed nanocrystalline TiO2 particles
inside the beads, which led to an enhanced
power conversion efficiency (PCE) of greater
than 10% [134].
In another approach, mesoporous
anatase spherical TiO2 architectures with high
surface areas of up to 116.5 m2g
−1 were
constructed by a simple urea-assisted
hydrothermal process and investigated as dye-
sensitized solar-cell electrodes. The DSSCs
showed enhanced light harvesting and a larger
amount of dye loading, which produced a
significantly higher overall light conversion
efficiency of 7.54% compared to a commercial
Degussa P25 TiO2 nanocrystalline electrode
(5.69%) [133]. Meanwhile, highly-crystallized
mesoporous TiO2 microspheres with surface
areas of up to 193.0 m2g
-1 were also created by
hydrothermal treatment of the titanium
diglycolate precursors and assembled into
DSSCs with a high light-to-electricity
conversion of 8.20%, indicating a 40% increase
in the conversion efficiency compared to the
standard Degussa P25 photoanode [131]
Furthermore, mesoporous TiO2 sub-
microspheres with a size distribution from 80 nm
to 3 μm were prepared using aerosol techniques
with a non-equilibrium density gradient [136].
Micrometer-sized hollow spheres of
TiO2 nanocrystals have the beneficial properties
of a large surface area, mesoporous morphology
on both external and internal surfaces, which
offers unique optical properties for light trapping
and scattering despite the large size of the
spheres [138-143]. For example, robust and size-
controllable hollow TiO2 microspheres (see
Figure 12) constructed through the assembly of
18 nm TiO2 nanoparticles have been synthesized
via a solvothermal reaction of titanium
isopropoxide by altering the concentration of
tetrabutylammonium hydroxide to control their
diameter. This technique is simple, highly
reproducible, and amenable to low cost, large-
scale synthesis [139]. Hollow TiO2 microspheres
can also be attained though a hydrothermal
process with TiOSO4 aqueous solution as a
precursor (see Figure 12) [140]. Moreover, TiO2
hollow spheres can be fabricated via a
hydrothermal method using carbon spheres as
hard templates through template-directed
deposition, followed by calcination in order to
remove the templates. The resulting TiO2 hollow
spheres exhibited superior photocatalytic activity
for the degradation of Rhodamine B, 2.9 times
greater than that of Degussa P25 [142].
Additionally, novel hollow TiO2 hemispheres
were produced through a combination of
chemical and physical synthesis routes, namely,
using a colloidal templating technique combined
with an RF-sputtering method (see Figure 12).
The hollow structure aided in electron transport,
provided a large surface area for enhanced dye
loading, and allowed penetration of even viscous
electrolytes. When formed into TiO2 photoanode
films for DSSCs, the properties of these particles
were found to enhance photoconversion
efficiency [141].
Microsphere TiO2 configurations
comprised of 1D or 2D TiO2 building blockings,
such as nanorods, nanotubes or nanosheets have
been fabricated [144-148]. For instance, single-
crystalline rutile TiO2 microspheres with
diameters ranging from 0.65 to 2.5 μm were
prepared by means of the assembly of 1D
nanorods with diameters of 10–15 nm and
shortest lengths of 250 nm through a simple
Figure 12. SEM (a, c, d, e) and TEM b, f) images of TiO2 hollow microspheres. (Reprinted with permission
from Reference [140] (a) and (b): Yu, J. G. et al., Adv. Funct. Mater. 2006, 16, 2035-2041. Reference [141]
(c) and (d): Yang, S. C. et al., Adv. Mater. 2008, 20, 1059. Reference [139] (e) and (f): Kim, Y. J. et al.,
Langmuir 2007, 23, 9567-9571. Copyright © American Chemical Society and Wiley-VCH).
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© 2011 Simplex Academic Publishers. All rights reserved.
surfactant-assisted solvothermal process in the
presence of the surfactant Span 80 [146].
Recently, the fabrication of a microsphere-like
TiO2 architecture consisting of rutile nanorods
with (110) exposed facets (see Figure 13)
through a simple hydrothermal method without
the use of templates was reported. A novel
outside-in ripening mechanism was proposed to
explain the formation of these TiO2 architectures
[148]. Moreover, hollow microspheres
constructed of TiO2 nanotubes were obtained via
a hydrothermal process in a mildly alkaline
solution, where the titanate nanotubes were
formed by rolling sheets with the assistance of
H2O2 (see Figure 13) [147]. Anatase TiO2
microspheres with exposed mirror-like plane
(001) facets (see Figure 13) were synthesized via
a facile hydrothermal process. The photoanode
composed of the as-formed TiO2 microspheres
showed an improved DSSC efficiency owing to
the superior light scattering effect of
microspheres and excellent light reflecting ability
of the mirror-like plane (001) facets [145].
5.4. TiO2 nanosheets
Among the low-dimensional
morphologies of TiO2, particular interest is being
devoted to TiO2 nanosheets, which are classified
as two-dimensional systems having a thickness
of a few atomic layers. Suprisingly, these
structures can be remarkably stable despite their
low dimensionality. TiO2 nanosheets exhibit
novel physical and chemical properties because
of their two-dimensionality, including very
intense and sharp UV absorption and
enlargement of the band gap energy [149-151].
For example, lepidocrocite structured (which has
a close structural relationship with anatase)
nanosheets, in which Ti ions are in a six fold
coordinated configuration, were computed to be
0.18 eV per unit formula higher in energy
compared to bulk TiO2 [152]. These first
principle calculations also predicted large
anisotropy in physical characteristics such as
optical, dielectric, and mechanical properties
[150,152,153]. Another important function of
TiO2 nanosheets has been their role as precursors
for the creation of other morphologies, such as
tubelike, boatlike, and wirelike TiO2 [154-156].
For example, well-ordered multilayer
films of TiO2 nanosheets were prepared on a
quartz-glass substrate using the layer-by-layer
deposition method, and their photocatalytic
oxidation ability was evaluated by
decomposition of gaseous 2-propanol and
bleaching of methylene blue dye under UV light
illumination. However, the photocatalytic
oxidation activity decreased with increasing
number of nanosheet layers, and the
investigation of the photochemical reactions on
the number of nanosheet layers revealed the
behavior of carriers generated in multilayered
systems. That is, migration of these carriers was
primarily limited to the two-dimensional plane,
and hopping between the nanosheets was
substantially restricted [149]. Additionally,
perpendicular TiO2 nanosheet films were
produced on a titanium metal sheet by
hydrothermal treatment with aqueous urea. The
large-area flat plane of the TiO2 nanosheet,
which was oriented vertically relative to the
substrate, was considered to be
thermodynamically stable, while the edge plane,
which had a thickness of several nanometers,
was expected to be thermodynamically unstable
(see Figure 14). Therefore, a perpendicular TiO2
nanosheet film is predicted to display
superhydrophilicity without UV irradiation
because the edge plane should contain a large
number of defects or dangling bonds as a result
of thermodynamic instability [157].
Figure 13. SEM images of TiO2 hollow microspheres based on (a), (b), (c) nanotubes; (d), (e) nanosheets; (f),
(g) nanorods. (Reprinted with permission from Reference [147] (a), (b) and (c):Tan, Y. F. et al., Langmuir
2010, 26, 10111-10114. Reference [145] (d) and (e): Zhang, H. M. et al., Chem. Commun. 2010, 46, 8395-
8397. Reference [148] (f) and (g): Sang, Y. et al., Nanoscale 2010, 2, 2109-2113. Copyright © American
Chemical Society and Royal Society of Chemistry).
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© 2011 Simplex Academic Publishers. All rights reserved.
Experimentally, anatase TiO2
nanosheets with largely exposed (001) facets
have been synthesized [154,158], and theoretical
studies have predicted that the (001) facets of
anatase TiO2 are much more reactive than the
thermodynamically stable (101) facets. This
prediction is based on the fact that (001) facets
contain high densities of under-coordinated Ti
atoms and very large Ti–O–Ti bond angles at the
surface, and thus, the formed anatase nanosheets
with exposed (001) facets exhibit a highly active
photocatalytic effect [3,152,159]. For instance,
anatase TiO2 nanosheets, with dominant (001)
facets fabricated by a facile hydrothermal route
in a Ti(OC4H9)4-HF-H2O mixed solution,
exhibited much higher photocatalytic activity in
the photocatalytic degradation of acetone as well
as enhanced performance of DSSCs compared to
anatase TiO2 nanoparticles with few (001) facets
and Degussa P25 TiO2 NPs (see Figure 15)
[151,160].
5.5. TiO2 nanotubes
Among the various forms of
nanostructured semiconductors, One-
dimensional (1-D) highly ordered architectures
(such as nanowires, nanorods, nanobelts,
nanoribbons, nanotubes) with high surface area-
to-volume ratios possess useful and unique
properties compared to that of their bulk
counterparts. The highly orientated nature of
these ordered 1-D nanostructures endows them
with excellent electron percolation pathways for
vectorial charge transfer between interfaces. For
Figure 14. (a, c) SEM images and (b, d) TEM images of the perpendicular TiO2 nanosheet films. (Reprinted
with permission from Reference [157], Hosono, E. et al., Langmuir 2007, 23, 7447-7450. Copyright ©
American Chemical Society).
Figure 15. SEM (a), TEM (b and c) and HRTEM (d) images of TiO2 nanosheets (a, c and d) and TiO2-
nanoparticles (b) films calcined at 450 0C. (Reprinted with permission from Reference [151], Yu, J. G. et al.,
Nanoscale 2010, 2, 2144-2149. Copyright © Royal Society of Chemistry).
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© 2011 Simplex Academic Publishers. All rights reserved.
example, the mobility of electrons in 1-D
nanostructures is typically several orders of
magnitude higher than that in semiconductor
nanoparticle films [161-164]. In comparison with
other 1-D morphologies, nanotubes provide a
larger interfacial area due to their external and
internal surfaces, which is beneficial for surface
area dependent applications. Studies on TiO2
have shown that vertically oriented nanotube
arrays are remarkably efficient when applied in
sensors, water splitting, dye-sensitized and solid-
state solar cells, and photocatalysis [80,165-169].
TiO2 nanotube arrays have been
synthesized by a variety of methods, including
template deposition (using nanoporous alumina
templates, ZnO nanorod templates, or organo-
gelator templates in sol–gel processes)
[87,88,170-172], electrochemical anodization
[79,81,173-175], and hydrothermal techniques
[165,176-181]. Among them, the cheapest and
most straightforward approach that leads to
ordered nanotubes is the anodization method,
which allows superior control over the resulting
tube diameter, tube length, and overall
morphology by the optimization of various
parameters such as the pH, concentration, and
composition of the electrolyte, as well as the
applied potential, time, and temperature of
anodization. That is, electrochemical
anodization, in which the equilibrium between
localized chemical dissolution with field-assisted
oxidation and dissolution reactions takes place,
produces precisely ordered nanoscale
architecture by self-assembly [79,80,182].
Generally speaking, there are four
development generations of nanotube arrays
prepared by electrochemical anodization. The
first report on the fabrication of highly ordered
TiO2 nanotube arrays was published in 2001 by
Gong and co-workers, in which they reported the
formation of nanotubes up to 0.5 mm length by
electrochemical anodization of Ti foil in HF
aqueous electrolyte [78]. This pioneering work
began a decade-long effort to refine the novel
technique, and a second generation of nanotube
array synthesis was triggered by the subsequent
work of Grimes and co-workers, in which the
nanotube array length was increased to 6.4 μm
by using an aqueous buffered electrolyte and by
proper control of the anodization electrolyte pH
to reduce chemical dissolution of the TiO2 during
anodization [79]. The third generation of TiO2
nanotube array synthesis yielded nanotube
lengths up to approximately 1000 mm using a
non-aqueous, polar organic electrolyte (such as
formamide, dimethyl sulfoxide, ethylene glycol,
or glycerol) [183]. Recently, synthesis of TiO2
nanotube arrays using non-fluoride electrolyte
has been reported, which may be considered as
the fourth synthesis generation [184,185].
To date, several reviews have
illuminated the many aspects of synthesizing
TiO2 nanotube arrays through anodization [79-
82]. By all accounts, the variation of
experimental conditions can be used to tune the
nanotube features [186-195], and double-walled
TiO2 nanotubes [196], multipodal TiO2
nanotubes [197], large diameter TiO2 nanotubes
(see Figure 16) [198], and several other
structures have been created. Regardless of
structure, TiO2 nanotube-based films must be
optimized for use in DSSCs. TiO2 nanotube
arrays are commonly grown in situ on opaque
titanium foils or sheets, but these are not suitable
for achieving high-efficiency DSSCs because the
incoming light is partially reflected by the
counter electrode and partially absorbed by the
counter electrode and iodine in the electrolyte
Figure 16. SEM images of (a) double-wall TiO2 nanotubes; (b) large diameter TiO2 nanotubes; (c) and (d)
multipodal TiO2 nanotubes. (Reprinted with permission from Reference [196] (a)Albu, S. P. et al., Adv.
Mater. 2008, 20, 4135. Reference [198] (b): Yoriya, S.; Grimes, C. A. Langmuir 2010, 26, 417-420. Reference
[197] (c), (d): Mohammadpour, A. et al., Acs Nano 2010, 4, 7421-7430. Copyright © Wiley-VCH and
American Chemical Society).
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before striking TiO2 nanotubes, leading to a loss
of ~25% of the incident solar energy [199].
Several strategies have been explored to
circumvent this problem. One of the most
straightforward solutions is to deposit titanium as
a thin film on an adequate substrate (such as
fluorine-doped tin oxide glass (FTO)) before
anodizing it. This is usually performed by
physical methods such as radio-frequency (RF)
or direct-current (DC) magnetron sputtering
[190,200-204]. Using such a deposition process,
effective conversion of sputtered titanium into
nanotube layers was achieved and DSSCs
containing these arrays yielded a power
conversion efficiency of 6.9% (see Figure 17)
[199]. Another approach for getting TiO2
nanotube arrays onto transparent conductive
electrodes involves growing the array on
titanium foil before transporting it to the
electrode. In this method, a large-area free-
standing TiO2 nanotube film was grown and
separated by a two-step anodization process
before it was transferred onto FTO glass and
anchored via a layer of TiO2 nanoparticle paste
[205-210]. However, in this case, the tube-ends
at the bottom of the array were closed, and the
interface between the TiO2 nanotube arrays and
the TiO2 nanoparticle layer caused near-UV light
absorption and front surface light reflection as
well as blockage of the diffusion of redox
reagents into the underlying TiO2 nanoparticles
coated on the collecting FTO substrate.
Figure 17. SEM images of (a) a 20-μm-thick Ti film deposited on FTO-coated glass. (b), (c) a 20-μm-long
TiO2 nanotube array from anodization of the as-deposited Ti film. (d) TEM image and selected area diffraction
(SAD) pattern of a region of a TiO2 nanotube fabricated from the as-deposited Ti film. (Reprinted with
permission from Reference [199], Varghese, O. K. et al., Nat. Nanotechnol. 2009, 4, 592-597. Copyright ©
Nature Publishing Group).
Figure 18. (a) Photograph and SEM images of (b) top, (c) side and (d) bottom views of the closed-end free-
standing TiO2 nanotube arrays; (e) and (f) SEM images of the opened-end free-standing TiO2 nanotube arrays.
(Reprinted with permission from Reference [211], Lin, C. J. et al., Mater. Chem. 2010, 20, 1073-1077.
Copyright © Royal Society of Chemistry).
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Accordingly, the caps were removed from the
closed bottom by immersing the as-prepared
free-standing TiO2 nanotube film in an oxalic
acid solution. Compared to the closed-end TiO2
nanotube-based DSSC, the opened-end TiO2
nanotube-based device exhibited an increase in
one-sun efficiency from 5.3% to 9.1%, yielding a
70% enhancement [211]. In addition to vertically
aligned nanotubes, a novel version of DSSC,
three-dimensional dye-sensitized solar cells (3D
DSSCs), was introduced to overcome the light
harvesting problem of TiO2 nanotube-based
electrodes. In this setup, titanium wires or
meshes are utilized instead of titanium foils or
sheets to fabricate TiO2 anodized nanotubes
[212-217]. For instance, Misra et al applied a
TiO2 nanotube-based wire as a working electrode
(see Figure19) and platinum wire as a counter
electrode in a DSSC, which achieved a
conversion efficiency of 2.78% under AM 1.5
simulated sunlight. The prototype device was
capable of achieving long distance transport of
photocurrent and harvesting of light from any
direction to generate electricity [214]. TiO2
nanotubes were also directly grown on spiral-
shaped titanium wire via anodization and
assembled into a 3D DSSC coupling to a special
platinized titanium wire counter electrode.
Unlike conventional flat DSSCs, this 3D DSSC
could easily hold liquid electrolyte due to
capillary forces that facilitated sealing of the cell.
These solar cells showed an enhanced energy
conversion efficiency of 4.1% under the AM 1.5
condition compared with that (3.2%) of the
backside illuminated TiO2 nanotube-based DSSC
having the same projected area [217].
Furthermore, Wang et al. developed a new type
of 3D DSSCs with double deck cylindrical Ti
meshes as the substrates. Here, one of the Ti
meshes is anodized to synthesize the self-
organized TiO2 nanotube photoanode layer in
situ. Another Ti mesh is platinized through
electrodeposition as the counter electrode. This
all-Ti solar cell exhibited the highest conversion
efficiency for 3D DSSCs of 5.5% under standard
AM 1.5 sunlight [213].
5.6. TiO2 nanowires/nanorods
As mentioned above, 1-D TiO2
nanocrystals have several advantages over their
spherical counterparts for environmental and
energy applications because of their high surface
area-to-volume ratio and enhanced charge
transport properties afforded by the decreased
number of intercrystalline contacts and elongated
structure. As important members of the 1-D
structure family, the development of
semiconductor nanowires/nanorods will provide
an opportunity to understand the physical
properties of all 1-D semiconductor
nanomaterials and facilitate the realization of
electrical, optical, and optoelectronic devices that
employ nanowires/nanorods-based materials.
TiO2 nanowires and nanorods are of particular
interest because of their demonstrated
applications in a wide variety fields including
DSSCs, water splitting, photocatalysis, and so
forth [218-224].
As with the other TiO2 structures,
nanowires and rods can be synthesized by a
broad set of methods including
solvothermal/hydrothermal [37,38,225-230],
chemical oxidation [77,231,232], sol-gel [233-
235], and physical vapor deposition techniques
[236-238]. For example, sol–gel processes
enable the growth of anatase TiO2 nanowires
from FTO substrates, which are sensitive to a
number of factors. The mechanisms of this
reaction can be tentatively described as follows.
First, Ti-oxide molecular clusters are bound to
Figure 19. Top view (a, b) and cross-sectional (c, d) SEM images of TiO2 nanotube arrays grown around a Ti
wire fabricated by anodization at 60 V for 12 h. (Reprinted with permission from Reference [214], Liu, Z. Y.;
Misra, M. Acs Nano 2010, 4, 2196-2200. Copyright © American Chemical Society).
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the pretreated FTO substrates, followed by a
series of subsequent hydrolysis steps, which
facilitate the axial growth of linear
macromolecules normal to the plane of the FTO
crystal faces. Calcination then produces a thin
film of anatase TiO2 nanowires tethered to the
FTO surface [234]. TiO2 nanorods were also
synthesized via a sol-gel method utilizing
Pluronic P-123 triblock copolymer, and the
resultant nanorod-based DSSC showed improved
performance with a Voc of 0.68 V, Jsc of 15.3
mA cm–2
, fill factor of 0.6, and efficiency of 6.2
%, due to the reduced intercrystalline contacts
between grain boundaries and stretched
structure, which facilitated greater electron
transport than a similar NP-based DSSC (a Voc
of 0.63 V, a Jsc of 10.9 mA cm–2
, a fill factor of
0.63, and an efficiency of 4.3%) [239]. In
addition to solar cells, dense and aligned TiO2
nanorod arrays have found utility as electrodes in
PEC studies for hydrogen generation from water
splitting. These structures were fabricated using
oblique-angle deposition on indium tin oxide
(ITO) conducting substrates, and overall water
splitting was observed with an applied over
potential at 1.0V (versus Ag/AgCl) with a photo-
to-hydrogen efficiency of 0.1% [240].
Furthermore, rod-based TiO2 anatase
nanocrystals were made by a simple one-step
solvothermal method, which prevented the
formation of coarse aggregates and provided
nanostructured films, in which the original
shapes and sizes of the constituent nanorods
were preserved. The as-prepared photoelectrodes
showed a higher surface area and a better
transparency with respect to those made with
commercial P25 TiO2, and the corresponding
DSSCs exhibited a 16.9 mAcm-2
photocurrent
density with a conversion efficiency of 7.9%
[241].
While 1-D polycrystalline TiO2
nanostructures suffer from deleterious electron
scattering or trapping at grain boundaries, the
synthesis of aligned single-crystalline TiO2
nanorod and nanowire films has attracted much
attention because the resultant films could offer
direct electrical pathways to increase the electron
transport rate, which in turn may improve the
performance of photovoltaic devices
[33,225,242-245]. For example, the electron
transport in single-crystalline nanowires is
expected to be several orders of magnitude faster
than percolation through a random
polycrystalline nanowire network. In practice,
DSSCs using single-crystalline anatase TiO2
nanowire electrodes formed in a network
structure by surfactant-assisted self-assembly
processes exhibited a high conversion efficiency
of 9.3% due to rapid electron transfer in the
Nanowires [246]. Single crystal rutile TiO2
nanowire arrays have also been grown vertically
from a TCO glass substrate along the (110)
crystal plane with a preferred (001) orientation
via a solvothermal process and showed an AM
1.5 photoconversion efficiency of 5.02% in a
DSSC (see Figure 20) [247]. Moreover, oriented
single-crystalline rutile TiO2 nanorod films
grown on FTO substrates were developed by a
facile hydrothermal treatment and achieved a
conversion efficiency of 3% when used as the
photoanode in a DSSC (see Figure 21) [244]. A
versatile method for the synthesis of single-
crystalline rutile phase TiO2 nanowires on
arbitrary substrates including FTO glass, glass
slides, ITO glass, Si/SiO2, Si (100), Si (111) and
glass rods has been developed. However,
optimization of the growth conditions only led to
well-aligned vertical arrays of TiO2 nanowires
on both FTO and glass substrates. The
application of vertical arrays of TiO2 nanowires
Figure 20. (a) and (b) SEM, (c) TEM images, and (d) XRD pattern of vertically oriented single crystal TiO2
nanowire array grown on FTO coated glass. (Reprinted with permission from Reference [247], Feng, X. J. et
al., Nano Lett. 2008, 8, 3781-3786. Copyright © American Chemical Society).
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on FTO as the photoanode in DSSCs was also
demonstrated, and an efficiency of ~3.0% was
achieved [243].
Recently, some colorful terms have
been adopted to describe several special 1-D rod-
like TiO2 structures, such as nanofiber [248-
253], nanospider [254], nanopillar [255-258],
nanospindle [259-261], nanoframe [262],
bipyramid [263], and rice-like (see Figure 22)
TiO2, [264,265]. TiO2 nanofibers can be readily
distinguished from TiO2 nanowires and nanorods
by their dimensions: nanowires and nanorods are
structures with diameters of tens of nanometers,
nanowires have undefined lengths, while
nanorods typically have lengths in the range of
ca. 1–100 nm. In contrast, electrospun nanofibers
have diameters of 100 nm or larger and lengths
of tens of microns or longer [248,252]. While the
above definitions represent the most common
application of the terms, these rules are
frequently broken in the literature.
5.7. TiO2 nanostructure transformation
Remarkably, under special synthesis
conditions, all of the TiO2 architectures can
transform into each other. It is even possible to
transform diversely shaped TiO2 nanocrystals via
a single synthesis method by adjusting
experimental conditions [266-271]. For example,
hydrothermally formed Na2Ti6O13 nanostructures
can be easily tuned by varying the experimental
parameters of temperature, reaction duration, and
Figure 21. SEM images of oriented rutile TiO2 nanorod film grown on FTO substrate (a) top view, (b) cross-
sectional view, (c) and (d) tilted cross-sectional views. (Reprinted with permission from Reference [244], Liu,
B.; Aydil, E. S. J. Am. Chem. Soc. 2009, 131, 3985-3990. Copyright © American Chemical Society).
Figure 22. SEM images of TiO2 (a) nanofibers; (b) and (c) nanospindles; (d) Rice grain-shaped. (Reprinted
with permission from Reference [248] (a) Zhang, W. et al., Small 2010, 6, 2176-2182. Reference [259] (b) Qiu,
Y. C. et al., Angew. Chem.-Int. Edit. 2010, 49, 3675-3679. Reference [261] (c) Qiu, Y. C. et al., Acs Nano
2010, 4, 6515-6526. Reference [264] (d) Nair, A. S. et al., Chem. Commun. 2010, 46, 7421-7423. Copyright ©
Wiley-VCH, American Chemical Society and Royal Society of Chemistry.
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© 2011 Simplex Academic Publishers. All rights reserved.
NaOH concentration in order to obtain
nanoplates, nanowires, and continuous nanowire
network films. The shapes of the Na2Ti6O13
nanostructures formed were preserved when
converted into H2Ti3O7 nanostructures by an ion-
exchange process, which served as desirable
precursors in the fabrication of corresponding
TiO2-based nanomaterials [272]. Again, a variety
of TiO2 nanostructures, including aligned
nanorods, nanoporous nanostructures, nanocubes
and diamond-shaped nanocrystals have been
prepared through an inorganic, acid-assisted
hydrothermal route, in which the nature and
concentration of the inorganic acid (HCl,
HNO3,or H2SO4) determined the morphology,
crystalline phase, composition, average grain
size, band gap, and microstructure of the
nanostructures (see Figure 23) [273].
Furthermore, TiO2 nanostructures with
controllable morphologies such as cubes,
spheres, and rods were synthesized by a simple
microwave irradiation technique. Transformation
of different morphologies was achieved by
tuning the pH and the nature of the medium or
precipitating agent [274].
In addition to control of the synthesis
parameters, there is another route for TiO2
nanostructure transformation, that is, one kind of
as-prepared architecture can be directly
converted into another type of morphology via a
post-synthesis treatment. In this respect, many
transformations are possible, including rods to
tubes [275], tubes to rods [276], tubes to
particles [277-281], tubes to wires [282], etc. For
instance, rutile TiO2 nanorods prepared by the
hydrolysis of titanium butoxide via a sol–gel
process were hydrothermally treated using
hydrochloric acid solution, which led to a novel
morphology of rutile TiO2 tapered nanotubes
with rectangular cross-sections. A plausible
anisotropic corrosion mechanism was proposed
for this transformation [275]. In addition,
hydrothermal treatment of TiO2 nanotube
suspensions under an acidic environment
resulted in the formation of single-crystalline
anatase nanorods with a specific crystal-
elongation direction. The nanotube suspensions
were prepared by a hydrothermal treatment of
P25 TiO2 nanoparticles in NaOH solution,
followed by mixing with HNO3 [276].
Interestingly, the conversion from nanotubes to
nanoparticles can be performed via treatment of
the as-anodized TiO2 nanotubes in concentrated
aqueous ammonium hydroxide solution prior to
calcination at elevated temperatures [280],
through a thermal annealing of the as-anodized
TiO2 nanotubes in ambient fluorine resulting
from the electrolyte residues of anodization
[277,279,281], or by a 1800C hydrothermal
process of the as-anodized TiO2 nanotubes
placed on a support to avoid direct contact with
the water in the reaction container (see Figure
24) [278].
5.8. TiO2 nanostructure combinations
Since every structure discussed thus far
possesses unique properties (such as the high
surface area of nanoparticles and high electron
mobility and transport rate of 1-D architectures),
the combination of two or more structures holds
promise to synergistically improve the
performance of photocatalytic devices. Several
combinations have attracted interest including
nanotube/ nanoparticle [283-286], nanorod/
nanoparticle [21, 30, 287-289], nanowire /
Figure 23. SEM and TEM images of TiO2 (a) aligned rutile ultralong nanorods; (b) a mixture of diamond-
shaped rutile and spherical anatase nanocrystals; (c) aligned rutile short nanorods; (d) porous rutile nanorods
with slit-like nanopores. (Reprinted with permission from Reference [273], Shen, L. M. et al., J. Phys. Chem.
C, 2008, 112, 8809-8818. Copyright © American Chemical Society).
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© 2011 Simplex Academic Publishers. All rights reserved.
nanoparticle [290-292], nanofiber/ nanoparticle
[293], nanotube/ nanorod [294], nanorod/
nanofiber [295], and nanobelt/ nanoparticle pairs
[28]. For example, a common combination of
TiO2 nanotubes and nanoparticles was realized
by treating the as-anodized nanotubes with TiCl4
solution hydrolysis (see Figure 25), which could
increase the nanotube surface area and modify
the cracks remaining from annealing, effectively
improving the conversion efficiency of the
nanotube-based DSSCs [91,296]. In addition,
anatase nanoparticles mixed with brookite
nanorods were obtained by thermal hydrolysis of
aqueous solutions of titanium bis (ammonium
lactate) dihydroxide in the presence of an
optimized concentration of urea, and they
exhibited a higher photocatalytic hydrogen
evolution activity than that of pure anatase
nanoparticles despite the lower surface area of
the former. This behavior can be due to the fact
that the flatband potential of the brookite
nanorods was shifted by 140 mV more
cathodically than the flatband potential of the
anatase nanoparticles, resulting in higher
photocatalytic activity [21]. Moreover,
nanoparticle/nanowire composites can possess
the advantages of both building blocks, namely,
the high surface area of nanoparticle aggregates
and the rapid electron transport rate and the light
scattering effect of single-crystalline nanowires.
Therefore, DSSCs made with these hybrid
structures exhibited an enhancement of power
Figure 24. Transformation from as-anodized TiO2 nanotubes into nanoparticles: (a) and (b) through a thermal
annealing of the as-anodized TiO2 nanotubes in ambient fluorine; (c) and (d) by a 180 C hydrothermal process;
(e) and (f) via a treatment in NH4OH solution prior to calcination. (Reprinted with permission from Reference
[277] (a) and (b): Alivov, Y.; Fan, Z. Y. J. Phys. Chem. C 2009, 113, 12954-12957. Reference [278] (c) and
(d):Yu, J. G. et al., J. Phys. Chem. C 2010, 114, 19378-19385. Reference [280] (e) and (f): Shokuhfar, T. et al.
J. Appl. Phys. 2010, 108. Copyright © American Chemical Society and American Institute of Physics).
Figure 25. Composition of TiO2 (a) and (b) nanotubes/nanoparticles; (c) and (d) nanotubes/nanorods; (e) and
(f) nanorods/nanoparticles. (Reprinted with permission from Reference [91] (a) and (b): Chen, C. C. et al., J.
Phys. Chem. C 2008, 112, 19151-19157. Reference [294] (c) and (d): Zhang, H. M. et al., Langmuir 2010, 26,
11226-11232. Reference [291] (e) and (f): Tan, B.; Wu, Y. Y. J. Phys. Chem. B 2006, 110, 15932-15938.
Copyright © American Chemical Society)
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© 2011 Simplex Academic Publishers. All rights reserved.
efficiency from 6.7% for pure nanoparticle cells
to 8.6% for the composite cell with 20 wt %
nanowires under 1 Sun AM1.5 illumination (100
mW/cm2) [291]. Additionally, an optimized
composite consisting of electrospun TiO2
nanofibers and conventional TiO2 nanoparticles
noticeably improved light harvesting due to an
increase in scattering without substantially
sacrificing the adsorption of dye molecules for
the DSSCs, meanwhile, the incorporation of
electrospun TiO2 nanofibers into conventional
TiO2 nanoparticles improved the electron
diffusion and transport. Thus, under the same
fabrication conditions and film thickness, the
DSSC prepared using the composite formed of
15wt% nanofibers and 85wt% nanoparticles
demonstrated 44% higher device efficiency
(8.8%) than that made using TiO2 nanoparticles
alone (6.1%) [293]. Recently, a novel
architecture made of a perpendicularly aligned
and highly ordered TiO2 nanorod/nanotube
(NR/NT) adjacent film was obtained by directly
anodizing a hydrothermally modified titanium
foil with an anatase layer (see Figure 25). The
resultant architecture consisted of a highly
ordered nanorod top layer that directly joined to
a highly ordered nanotube array bottom layer,
and the photocatalytic performance toward water
and organic compounds demonstrated that
photocatalytic oxidation of water of the NT
photoanode was almost 1.5 times greater than the
NR/NT photoanode. However, the photoactivity
of the NR/NT photoanode toward glucose
oxidation was nearly 1.4 times higher than that
of the NT photoanode. In other words, the
NR/NT photoanode possesses the characteristic
of selective photocatalytic oxidation toward
organics (i.e., glucose), which is beneficial for
applications in environmental remediation and
analytical determination of organic pollutants,
where selective oxidation of organics is highly
desirable [294].
5.9. TiO2 hierarchical structures
Recently, hierarchically structured
materials with various morphologies have
attracted great attention. The concept of
hierarchically organized materials, especially
multilevel 3-D organization based on a host
macrostructure, allows the 3-D organization
necessary for fast mass transport. On this host
macrostructure, a secondary guest micro- and/or
nanoscale substructure is built in order to take
advantage of the properties of nanometer-sized
building blocks and micron- or submicron-sized
assemblies. The formation of hierarchical
structures is generally considered to be a self-
assembly process, in which building blocks, such
as nanoparticles (0D), nanorods or nanotubes
(1D), and nanosheets (2D) self-assemble into
regular higher level structures [162,297]. Much
effort has been devoted to assembling TiO2
building blocks into 3-D ordered superstructures
or complex functional architectures. For
instance, special architectures based on TiO2
nanorods, such as nanotrees [298], microflowers
[299-304], or microspheres [305], and branched
nanostructures based on TiO2 nanowires [306],
chestnut-like morphologies with TiO2 nanopins
[307], hierarchical hollow microspheres or
branched morphologies [308], assembled by
TiO2 nanotubes, microspheres organized from
TiO2 nanoparticles, TiO2 nanosheets built from
microflowers [309,310], microspherulites
[311,312], and mesoporous anatase layers
coupled to a three-dimensional photonic crystal
architecture [313], have been successfully
prepared. To date, many methods for the
fabrication of hierarchical geometries are applied
and performance improvements have been
achieved for solar cell application, despite the
fact that the exact assembly mechanisms remain
elusive.
Recently, a novel forest-like
architecture consisting of hierarchical assemblies
of nanocrystalline particles of anatase TiO2,
morphologically resembling a tree, were grown
on FTO substrates via pulsed laser deposition
(PLD) at room temperature by ablation of a Ti
target in a background O2 atmosphere (see
Figure 26). The resulting architecture was
proposed to be effective at hampering electron
recombination and controlling mass transport in
the mesopores, and, thus, achieved a 4.9%
conversion efficiency in a DSSC [298].
Moreover, ordered tree-like rutile TiO2
nanoarrays were developed through a simple,
one-pot synthesis approach involving the
reaction of a titanium surface with the vapor
generated from a hydrochloric acid solution in a
hydrothermal process. In addition to tree-like
structures, seaweed-like TiO2 nanoarrays were
grown on the surface of a titanium sheet by
hydrogen peroxide sculpting at low temperature
and exhibited an efficiency of 3.2% for the
corresponding DSSC with low recombination
rates and long electron life times (see Figure 26)
[314]. A chestnut-like morphology with rutile
nanopin TiO2 was also developed from large
single crystals of octahedron-like anatase TiO2
by adjusting the concentration of sodium dodecyl
sulfate in the hydrothermal route using aqueous
titanium trichloride solutions as precursors (see
Figure 26) [307]. Furthermore, TiO2
nanostrawberry rutile films were fabricated on a
large scale from aqueous solution via a sol-gel
method with seeded growth at low temperature
without any pressure equipment. The behaviors
of reversible switching between
superhydrophilicity and superhydrophobicity
JOURNAL OF NANOSCIENCE LETTERS
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© 2011 Simplex Academic Publishers. All rights reserved.
were observed on the film, due to the special
structure and native wettability of the TiO2 (see
Figure 26) [315].
6. Summary
Over the past years, the numerous
efforts devoted to TiO2 nanomaterials have
enriched knowledge of their synthesis,
properties, modification, and applications.
Continuing breakthroughs in synthesis
technology has introduced new TiO2
nanostructures, which exhibit the well-known
quantum-confinement effect and size/shape-
dependent properties as well as unique optical,
electronic, thermal, and structural properties,
while still possessing high photocatalytic
activity. Exploitation of novel architectures
including nanorods, nanotubes, and nanowires as
well as mesoporous and hierarchical structures
will continue, accompanied by progress in
manufacturing techniques. Therefore, TiO2 will
likely play an important role in the remediation
of environmental pollutants and in the search for
renewable and clean energy devices in the near
future.
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Cite this article as:
Zhiqun Lin et al.: Nanostructured TiO2 architectures for environmental and energy applications.
J. Nanosci. Lett. 2012, 2: 1